1. Technical Field
This disclosure relates to temperature compensation of metal resistors embodied in semi-conductor chips. More specifically, this disclosure relates to circuits for generating a temperature compensating reference voltage, as well as layouts and trimming techniques for such circuits.
2. Description of Related Art
Metal resistors are used in semi-conductor chips for a variety of purposes. In some applications, the metal resistor serves to sense an operating parameter of the circuit, such as the amount of current that is being delivered to a battery while it is being charged and/or removed from it while it is being used.
The resistance of metal resistors typically fluctuates as a function of temperature. Such changes typically occur because of heat generated by the metal resistor, by other components, and/or by other sources. These temperature-dependent deviations in the resistance of the metal resistor can adversely affect the accuracy of its sensing and, in turn, the performance of related circuit functions.
One approach to addressing this issue has been to apply a temperature-compensating voltage to an appropriate point in the circuit to compensate for variations in the resistance of the metal, resistor as a function of temperature. As the resistance increases because of increasing temperature, so does the compensating voltage. When applied appropriately, the temperature-compensating voltage can reduce errors that would otherwise be caused by temperature deviations in resistance.
One typical approach for generating a temperature-compensating voltage is to use what is known as a delta Vbe voltage reference circuit. Such a circuit generates a voltage that varies in proportion to absolute temperature, i.e., a proportional-to-absolute-temperature (“PTAT”) voltage. Unfortunately, PTAT voltages typically have a temperature-dependent curve which, when extrapolated, reaches zero volts at 0 Kelvin. The resistance of metal resistors, on the other hand, typically has a temperature-dependent curve which, when extrapolated, reaches zero ohms other than at 0 Kelvin. These differences in zero crossing locations can reduce the ability of a PTAT voltage to accurately compensate for deviations in the resistance of a metal resistor caused by temperature variations.